a previous change decreased the size of a zone object and it seems
that sometimes the OS report more memory used even though there
is less zone allocation.
Review URL: https://chromiumcodereview.appspot.com/9443019

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10814 ce2b1a6d-e550-0410-aec6-3dcde31c8c00